This invention relates to television horizontal oscillator frequency control (AFPC) arrangements in which the loop filter is switched and in which the switching device creates an offset which perturbs the loop operation.
The raster of a television display is scanned in the vertical and horizontal directions at a rate controlled by vertical and horizontal synchronizing signals, respectively, which are associated with the video to be displayed. Since the composite video may contain noise components, it is customary to provide the television device with noise immunity in the form of a phase-lock loop, in which a controllable horizontal-frequency oscillator is coupled in a feedback loop and a phase-sensitive detector and a low-pass filter, whereby the phase detector produces pulses representative of the difference in phase between the controlled oscillator and the horizontal synchronizing pulses associated with the video. The pulses produced by the phase detector are filtered by the low-pass filter, the output of which is coupled to the control input of the oscillator, and the oscillator rate is thereby maintained equal to the average synchronizing pulse rate.
It will often be desired to supply the television display device with composite video from a video playback device or tape recorder. Such video tape recorders commonly use a plurality of reproduction heads, each of which is mechanically scanned across the tape. In one common scheme, two heads are used, which alternately scan the tape for a duration equal to that of a vertical field. In order to avoid loss of, or breaks in, the displayed information, the succeeding field scanning is commenced by the second head substantially concurrently with the end of scanning in the first head. However, slight differences in tape tension or in the dimensions of the mechanical tape transport acting on the tape during playback compared with the tension and dimensions when the tape was recorded results in differences in the time between succeeding horizontal synchronizing pulses in the information played back as compared with that recorded, especially during the interval in which switchover between the scanning heads occurs. This results in a discontinuity or step in the phase of the horizontal synchronizing pulses available for synchronizing the horizontal oscillator and for controlling the raster. The step in phase normally occurs about five horizontal lines before the end of a vertical scanning interval.
Such a step change of phase will cause the phase-lock loop to control the horizontal oscillator in such a manner as to cause the oscillator phase to change and accommodate itself to the altered phase of the synchronizing signals. However, the gain characteristics of the low-pass filter of the phase-lock loop normally prevents the controlled oscillator from changing at a rate great enough to provide total synchronization between the controlled oscillator and the synchronizing signals before the end of the vertical blanking interval. This may result in a distortion of the displayed video.
U.S. Pat. No. 3,740,489 issued June 19, 1973 in the name of D. Willis describes an external switch by which the user may change the characteristics of the low-pass filter in the phase-lock loop when it is desired to use a tape recorder as the video source. This alters the gain characteristics of the phase-lock loop, allowing the oscillator to change phase or to slew towards the new phase at a greater rate. However, the user may forget to throw the switch, thereby permitting the aforementioned video distortion, or may forget to return the switch to its normal condition when receiving transmitted or broadcast signals, which will make the television receiver horizontal synchronization more sensitive to noise.
It is also known from U.S. Pat. No. 3,846,584, issued on Nov. 9, 1974 in the name of Itoh, et al., to completely disconnect the low-pass filter from the phase-lock loop for a predetermined limited period of time by the use of a switch. U.S. Pat. No. 4,144,544 issued Mar. 13, 1979 to Fernsler illustrates the use of the collector-emitter path for a bipolar transistor as the switching device. It is also known from U.S. Pat. No. 4,093,963 issued June 6, 1978 to Uchida to use a pair of complementary bipolar transistors connected in an antiparalleled fashion as the switching device.
A bipolar transistor when used to switch bidirectional currents will have unequal voltage drops for the two directions of conduction. The problem is not solved by antiparalleling of the transistors, since the base drive is unidirectional, the transistors in general will not have matching characteristics and because of the asymmetrical duty cycle. Consequently, during the ON time of the transistor switch, there is a net rectification of the bidirectional current produced by the phase detector and applied to the switched filter. This rectified signal produces a charge on the filter capacitors which is not dependent upon the relative phase of the horizontal oscillator and the horizontal synchronizing signals, but which depends instead upon the ratio of the ON to the OFF time of the switch. Thus, each transition from ON to OFF of the switching transistor which changes the time constant of the AFPC loop filter introduces a transient voltage to which the filter capacitors respond and which causes unwanted slewing of the horizontal oscillator, which may cause bending of vertical lines as displayed on the kinescope.